Strong dependence of the vertical charge carrier mobility on the π-π stacking distance in molecule/graphene heterojunctions
Due to mechanical flexibility and low cost, the heterojunctions consisted of graphene and small organic molecules are regarded as promising candidate materials for the vertical organic field-effect transistors (VOFET), where the charge carrier mobility perpendicular to graphene plane is crucial to their performances. Herein, through density functional simulations we find that the vertical charge carrier mobility of the heterojunctions can be greatly adjusted by tuning their π-π stacking distances. For the 6,13-dichloropentacene (DCP)/graphene heterojunctions, with the distance between the first DCP layer and graphene decreasing to below 2.4 Å, the vertical electron mobility between DCP layers is improved dramatically while the vertical hole mobility is greatly reduced. The strong dependence of vertical charge carrier mobility on the distance between first molecular layer and substrate for smaller values than typical π-π stacking distance (3.3~3.8 Å) was also observed in the perylenetetracarboxylic dianhydride (PTCDA)/graphene and DCP/hexagonal-BN heterojunctions, where the trendency is very different to that of DCP/graphene heterojunction. Our simulation results enable us a new strategy to tune the vertical charge transport properties in molecule/graphene heterojunctions, which provides insights into developing efficient VOFET.